In the related art, a sheet binding device that binds an edge part of a sheet bundle by using an adhesive tape is known. The sheet binding device is provided with a bundle forming part and a tape mounting part. The bundle forming part forms a sheet bundle by stacking a plurality of sheets. The bundle forming part forms a side part of the sheet bundle in a stepwise shape in order to secure a surface area when the tape is mounted thereon. The tape mounting part binds the sheet bundle by mounting the adhesive tape on the edge part of the sheet bundle. The tape mounting part is provided with a tape holding part that holds the adhesive tape. The tape mounting part is provided with a first roller and a second roller that are opposite to each other in a sheet bundle thickness direction. The adhesive tape is peeled from the tape holding part by inserting the sheet bundle shifted in the stepwise shape toward the adhesive tape held by the tape holding part. Thereafter, the sheet bundle enters between the first roller and the second roller together with the adhesive tape, and the adhesive tape adheres to the edge part of the sheet bundle.
However, the following problem may occur depending on a relationship between a thickness of the sheet bundle and a roller gap between the first roller and the second roller. For example, when the roller gap therebetween is too narrow with respect to the thickness of the sheet bundle, the sheet bundle cannot enter between the first roller and the second roller. On the other hand, for example, when the roller gap is too wide with respect to the thickness of the sheet bundle, the adhesive tape cannot sufficiently adhere to the edge part of the sheet bundle.